As the density of devices, such as resistors, capacitors, and transistors, in an integrated circuit is increased, the distance between the signal carrying conductors is decreased, and the capacitive coupling between the conductors is increased. Several problems result from the increased capacitive coupling. First, the increased capacitive coupling reduces the rate at which information can be transferred along each of the signal carrying conductors. Second, the increased capacitive coupling between the signal carrying conductors reduces the noise margin on the conductors. In the worst case, a signal on one signal carrying conductor is capacitively coupled to an adjacent signal carrying conductor, and the information on the adjacent conductor is destroyed. Since it is desirable to avoid destroying information, it is also desirable to reduce the capacitive coupling between the signal carrying conductors of an integrated circuit.
In an integrated circuit, decreasing the dielectric constant of an insulator that separates two adjacent signal carrying conductors reduces the capacitive coupling between the two adjacent signal carrying conductors. Silicon dioxide is the most commonly used insulator in the fabrication of integrated circuits and has a relatively high dielectric constant of about four. Carbon dioxide has a smaller dielectric constant than silicon dioxide, so replacing silicon dioxide with carbon dioxide reduces the capacitive coupling between the two adjacent conductors. Unfortunately, the thermal conductivity of carbon dioxide is much less than the thermal conductivity of silicon dioxide. This lower thermal conductivity causes a reduction in the rate at which heat is conducted away from an integrated circuit chip that employs a carbon dioxide insulator, which can result in the catastrophic failure of the integrated circuit.
Air has a dielectric constant of one, which is less than the dielectric constant of carbon dioxide and much less than the dielectric constant of silicon dioxide. Replacing silicon dioxide with air in an integrated circuit reduces the capacitive coupling between signal carrying conductors. Air bridge structures, which are structures consisting primarily of signal carrying conductors surrounded by air in an integrated circuit, are fabricated to reduce the dielectric constant in the conductive structures of an integrated circuit. Unfortunately, since, in an air bridge structure, the signal carrying conductors are no longer embedded in a layer of silicon dioxide, the structural integrity of the integrated circuit is decreased. This problem is especially significant when an integrated circuit fabricated using air bridge structures is packaged as a flip chip,
For these an other reasons there is a need for the present invention.